Generally, a light-emitting diode (LED) is generally formed of a first conductive layer having a first conductivity type, an active layer, and a second conductive layer having a second conductivity type, thereby forming a diode that will generate light when it is forward-biased. Different colors of LEDs may be created by using materials with different band gaps. The active layer typically emits light from both sides of the light-emitting layer and is propagated in all directions. In practice, however, applications frequently only require light emitted from a single side and along a specific direction, and because light is emitted from both sides and in all directions, some light energy is lost.
In an attempt to increase the amount of light emitted from a single side of the LED device, a reflective layer has been formed between the substrate and the LED structure. The reflective layer comprises a metal reflective material that acts to reflect light emitted from the LED device on the substrate side back towards the light-emitting face of the LED device, thereby increasing the light efficiency of the LED device.
While the reflective metal layer helps to increase the light emitted on a single side of an LED device, the light emitted is still generally propagated outwards in all directions over a 180° range. This propagation of light emitted in all directions is undesirable in some applications that require the light to be emitted along a specific direction, such as to a set of lenses, or the like.
Another attempt at enhancing the light output from an LED device is to roughen the light-emitting surface. An LED device having a smooth surface will exhibit a higher degree of total internal reflection, wherein light is reflected back toward the active layer rather than being emitted. To reduce the total internal reflection, the surface of the LED device may be roughened. The surface roughness is typically controlled by the metal organic chemical vapor deposition process when forming the LED device or by an etch process after the LED device has been formed. While the surface roughening increases the light output, creating a good ohmic contact on the rough surface is difficult.
Yet another method of enhancing light output from an LED device involves the formation of nanorods. In this method, the LED device comprises many nano-scale light-emitting rods that extend vertically upward from a substrate. The light output from a nanorod structure, however, is still reduced by total internal reflection because a large fraction of the light produced by the nanorods is incident on the nanorod sidewalls at an angle larger than the critical angle.
As a result, there is a need for an LED device having an increased light efficiency.